In the process of manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to perform impurity doping by an ion implantation process and a subsequent annealing process. The ion implantation process is a technique for causing ionized impurity elements such as boron (B), arsenic (As) and phosphorus (P) to collide against the semiconductor wafer with high acceleration voltage, thereby physically implanting the impurities into the semiconductor wafer. The implanted impurities are activated by the annealing process. When annealing time in this annealing process is approximately several seconds or longer, the implanted impurities are deeply diffused by heat. This results in a junction depth much greater than a required depth, which might constitute a hindrance to good device formation.
In recent years, attention has been given to flash lamp annealing (FLA) that is an annealing technique for heating a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate the surface of a semiconductor wafer with a flash of light, thereby raising the temperature of only the surface of the semiconductor wafer implanted with impurities in an extremely short time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of the xenon flash lamps is shorter than that of conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with a flash of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that flash irradiation in an extremely short time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time with the xenon flash lamps allows only the activation of impurities to be achieved without deep diffusion of the impurities.
The implantation of high-energy ions by the ion implantation process results in the induction of a large number of defects in silicon crystals of the semiconductor wafer. Such defects are prone to be induced in positions slightly deeper than an ion-implanted layer. During the annealing process subsequent to the ion implantation, it is desirable to perform the recovery of the induced defects as well as the activation of impurities. For such recovery of the defects, the time for annealing process may be made longer. This, however, presents a problem such that the impurities implanted as mentioned above are diffused more deeply than are required.
For this reason, a flash lamp annealing technique which performs additional irradiation with light with a relatively low emission output after a peak of the emission output is passed is disclosed in Japanese Patent Application Laid-Open No. 2009-260018. According to the technique disclosed in Japanese Patent Application Laid-Open No. 2009-260018, the temperature of the front surface of a semiconductor wafer is raised to a treatment temperature, and is thereafter maintained at the treatment temperature for approximately several milliseconds or more by the additional irradiation with light. This allows the heating of the semiconductor wafer in a position slightly deeper than the front surface to some extent, thereby accomplishing not only the activation of the impurities but also the recovery of the induced crystal defects.